How to Raise Koi Fry: First 30 Days Feeding, Water Care, and Culling
To raise koi fry successfully, incubate fertilized eggs on spawning mops with gentle aeration at 68°F to 72°F (20°C–22°C) until hatching at 3 to 4 days. Do not feed free-swimming fry until they absorb their yolk sac on day 3. Then, transition them from boiled egg yolk suspension and infusoria to freshly hatched baby brine shrimp (Artemia) and finely powdered fry meal across days 4 to 30, performing daily water changes to remove ammonia.
Key Takeaways
- Fertilized eggs hatch in 3 to 5 days; unfertilized white eggs must be removed to avoid fungal blooms.
- Fry survive on their internal yolk sac for the first 48 to 72 hours; never feed before they swim horizontally.
- Live baby brine shrimp (Artemia nauplii) provide the essential lipids and enzymes needed for rapid skeletal growth.
- First culling (senbetsu) occurs between days 30 and 45 to cull deformed spines and low-pigment fry.
Hatching Phase and Yolk Sac Absorption (Days 1–3)
After spawning, transfer egg-laden nylon mops immediately into a dedicated nursery tank with mature water and low-volume bubbling aeration. If left in the main pond, adult koi will consume tens of thousands of eggs within hours. Unfertilized eggs turn opaque white within 24 hours and will develop saprolegnia fungus if not treated with a mild methylene blue bath.
Eggs hatch between 72 and 96 hours depending on water temperature. Newly hatched fry (kego) resemble tiny transparent splinters with oversized black eyes. They attach themselves to tank walls with adhesive head glands and absorb their ventral yolk sacs. Do not add food during this phase, as unconsumed food will rot and destroy water quality.
First Food and Live Micro-Plankton Diets (Days 4–14)
On day 3 or 4, fry fill their swim bladders at the surface and begin horizontal free-swimming. Their mouths are micro-sized (under 100 microns). The classic starter food is hard-boiled egg yolk squeezed through a 50-micron mesh cloth into water, or live infusoria and rotifer cultures.
By day 5, transition rapidly to live, freshly hatched baby brine shrimp (Artemia nauplii). Brine shrimp trigger natural hunting predatory reflexes, provide high unsaturated fatty acids, and do not foul tank water as rapidly as prepared powders. Feed small portions 4 to 6 times daily.
Transitioning to Powders and the First Cull (Days 15–30)
By the second week, fry develop visible pectoral and caudal fins and begin searching the bottom. Introduce micro-pulverized high-protein spirulina fry meal and decapsulated brine shrimp eggs. Because heavy feeding produces heavy waste, perform 20% gentle daily water siphoning using fine net sieves over hoses to avoid sucking up fry.
At day 30, fry measure roughly 0.5 to 0.75 inches (12–20 mm). This marks the start of the first selection process (senbetsu). In Japanese breeder tradition, fry with spinal kinks, missing gill plates, or washed-out muddy patterns are culled, allowing remaining high-potential prospects adequate tank space, oxygen, and food to thrive.
Daily Routine for the First 30 Days of Koi Fry Care
- Days 1–3: Maintain gentle bubble aeration at 70°F (21°C); do not add any food.
- Days 4–7: Feed micro-strained egg yolk and infusoria 4 times daily; begin daily bottom debris siphoning.
- Days 8–18: Feed freshly hatched baby brine shrimp (Artemia) 5 times daily; conduct 20% water changes daily.
- Days 19–30: Introduce 100–200 micron powdered fry crumb; monitor ammonia and nitrite closely.
- Day 30+: Perform first selection (senbetsu) in a shallow white dish to remove deformed fry.
Recommended Reading & Resources
Explore our comprehensive guide on How Many Times A Day Should You Feed Koi Fish? to learn more about understanding feeding frequency differences between tiny fry and adult fish.
Explore our comprehensive guide on What Are The 3 Varieties Of Koi Fish? to learn more about evaluating hereditary color patterns during early fingerling development.
Common Questions
How many eggs does an adult female koi lay?
A healthy, mature female koi lays between 50,000 and 100,000 eggs per kilogram of body weight, meaning a large female can easily release over 300,000 eggs in a single spring spawn.
Why do so many koi fry die in the first two weeks?
The primary causes of massive fry mortality are starvation due to food particles being too large for tiny mouths, ammonia toxicity from overfeeding egg yolk, or getting pulled into powerful filter intakes.
Can you raise koi fry in a standard aquarium?
You can hatch eggs and raise fry for the first 2 to 3 weeks in a 40–50 gallon aquarium, but within 6 weeks, hundreds of growing fry require a large stock tank or mud pond to prevent severe stunting.